Emergency telephone services, such as 911 calls to a Public Safety Answering Point (PSAP), are an important public safety mechanism for connecting distressed callers to emergency services that are needed in often dire circumstances. Such 911 calls or distress calls (herein referred to collectively as “emergency calls”) often rely on Voice over Internet Protocol (VoIP) telephony nodes to facilitate a connection between a distressed caller and one or more PSAPs. Unfortunately, VoIP telephony nodes are constantly under threat because of their accessibility via the Internet. For example, malicious attackers may target the VoIP nodes with malformed Session Initiation Protocol (SIP) messages, exploit security holes, and/or flood the VoIP network with fake or spoofed calls. These types of Telephony Denial of Services (TDoS) and Distributed Denial of Services (DDoS) attacks on critical application servers can have severe repercussions on the availability of emergency services to a distressed caller. This is particularly true when PSAPs are flooded and over loaded with emergency calls without enough emergency responders available to answer the calls.
The resulting inability to reach emergency services creates potentially life-threatening situations for distressed callers in need of assistance. A need therefore exists to mitigate malicious attacks on emergency services over IP networks, including VoIP networks, which are generally architected as an Internet Protocol Multimedia Subsystem (IMS) network (or a variant thereof) involving multiple nodes that provide specific functionalities. A typical IMS network comprises multiple nodes that offer functionalities based on developed standards and protocols as disclosed and incorporated by reference below. Accordingly, a solution to mitigate telephony attacks is needed that does not interfere with existing standards but rather uses available protocols to interoperate with various IMS nodes. Therefore, a need further exists to mitigate telephony attacks using a commonly used VoIP protocol, such as SIP.